This invention relates generally to load responsive fluid control valves and to fluid power systems incorporating such valves, which systems are supplied by a single fixed or variable displacement pump. Such control valves can be used in a multiple load system, in which a plurality of loads is individually controlled under positive and negative load conditions by separate control valves.
In more particular aspects this invention relates to direction and flow control valves capable of controlling simultaneously a number of loads under both positive and negative load conditions.
In still more particular aspects this invention relates to direction and flow control valves capable of controlling simultaneously multiple positive and negative loads, which while controlling a negative load interrupt pump flow to the motor providing the motor inlet with fluid from the pressurized system exhaust.
Closed center load responsive fluid control valves are very desirable for a number of reasons. They permit load control with reduced power losses and therefore, increased system efficiency and when controlling one load at a time provide a feature of flow control irrespective of the variation in the magnitude of the load. Normally such valves include a load responsive control, which automatically maintains pump discharge pressure at a level higher, by a constant pressure differential, than the pressure required to sustain the load. A variable orifice, introduced between pump and load, varies the flow supplied to the load, each orifice area corresponding to a different flow level, which is maintained constant irrespective of variation in magnitude of the load. The application of such a system is, however, limited by several basic system disadvantages.
Since in this system the variable control orifice is located between the pump and the load, the control signal to a pressure regulating throttling device is at a high pressure level, introducing high forces in the control mechanism. Another disadvantage of such a control is that it regulates the flow of fluid into the motor or actuator and therefore does not compensate for fluid compressibility and leakage across both motor and valve. A fluid control valve for such a system is shown in U.S. Pat. No. 3,488,953 issued to Hausler.
The valve control can maintain a constant pressure differential and therefore constant flow characteristics when operating only one load at a time. With two or more loads, simultaneously controlled, only the highest of the loads will retain the flow control characteristics, the speed of actuation of the lower loads varying with the change in magnitude of the highest load. This drawback can be overcome in part by the provision of a proportional valve as disclosed in my U.S. Pat. No. 3,470,694, dated Oct. 7, 1969 and also in U.S. Pat. No. 3,455,210 issued to Allen on July 15, 1969. However, while these valves are effective in controlling multiple positive loads they utilize a controlling orifice located between the pump and the motor during positive load mode of operation and therefore control the fluid flow into the fluid motor instead of controlling fluid flow out of the fluid motor. These valves, also while effective when controlling positive loads, when operating negative loads connect system pump with actuator inlet, unnecessarily using the pump flow and subjecting actuator to increased loads.
This drawback can be overcome in part by provision of fluid control valves was disclosed in U.S. Pat. No. 3,807,447 issued to Masuda on Apr. 30, 1974. However, while these valves utilize actuator exhaust fluid for actuator inlet flow requirement, when controlling negative loads they regulate actuator inlet pressure by bypassing fluid to a downstream load circuit. Masuda's valves and their proportional control system are based on a series type circuit in which excess fluid flow is successively diverted from one valve to the other and in which loads arranged in series determine the system pressure. In such a system, flow to the last valve operating a load must be delivered through all of the bypass sections of all of the other system valves, resulting in higher fluid throttling loss. These valves are not adaptable to simultaneous control of multiple loads in parallel circuit operation since they do not provide a system load control pressure signal to the pump flow control mechanism. Also, the controls of these valves respond to the pressure differential due to flow across a variable orifice therefore varying the area of flow between the actuator inlet and actuator outlet with variation in the system pressure during control of negative load.